Apparatus for stopping a rotatably driven machine at a predetermined position

ABSTRACT

A rotatably driven sewing machine is enabled to be stopped at a predetermined position through the use of a pilot generator which detects the speed of rotation of the machine and a position detecting device. The pilot generator includes a rotor and stator each of which have a plurality of magnetic poles. The position detecting device includes an auxiliary shaft which is connected directly to the main shaft of the machine and an annular segment thereon which includes an electrical conductor and insulating members. A plurality of brushes make contact with the annular segment and will contact the insulating members when the center of the magnetic poles of the rotor of the pilot generator are aligned with that of the stator.

United States Patent Oke et a].

[ 1 Mar. 7, 1972 [54] APPARATUS FOR STOPPING A ROTATABLY DRIVEN MACHINEAT A PREDETERMINED POSITION [72] Inventors: Syoichi Oke; TakanoriKobayashi, both of Nagoya, Japan [73] Assignee: Mitsubishi ElectricCorporation, Tokyo,

Japan [22] Filed: June 29,1970

[21] App]. No.: 50,820

[30] Foreign Application Priority Data July 10, 1969 Japan ..44/54758[52] US. Cl ..l92/l46, 192/18 B, 192/147, 112/219 A [51] Int. Cl. Fl6d71/02, D05b 69/26 [58] FieldofSearch ..192/18 B, 146, 147; 112/219A [56]References Cited UNITED STATES PATENTS Primary Examiner-Allan D. HermannAttorney-Oblon, Fisher and Spivak [57] ABSTRACT A rotatably drivensewing machine is enabled to be stopped at a predetermined positionthrough the use of a pilot generator which detects the speed of rotationof the machine and a position detecting device. The pilot generatorincludes a rotor and stator each of which have a plurality of magneticpoles. The position detecting device includes an auxiliary shaft whichis connected directly to the main shaft of the machine and an annularsegment thereon which includes an electrical conductor and insulatingmembers. A plurality of brushes make contact with the annular segmentand will contact the insulating members when the center of the magneticpoles of the rotor of the pilot generator are aligned with that of thestator.

5 Claims, 6 Drawing Figures PATENTEBMAR 71912 3. 647, 039

' sum 1 0F 5 INVENTOR SYOIcHI OKE TA KANORI KO AYASm BY 06km, 3mm WATTORNEY PATENTEDMAR 7 i972 SHEET 2 [1F 5 INVENTOR SYOICHI OKE TAKANOmKOBIWASI-H BY Wyn- 5%4 ATTORNEY PATENTEDMAR 7 I972 sum h or 5 INVENTOR5Y \CH| OKE TAKANORI KOBA{A5H| ATTORNEY APPARATUS FOR STOPPING AROTATABLY DRIVEN MACHINE AT A PREDETERMINED POSITION BACKGROUND OF THEINVENTION 1, Field of the Invention This invention relates generally toan apparatus for stopping a rotatably driven machine at a predeterminedposition, and more particularly to an apparatus for stopping a rotatablydriven machine at a predetermined position wherein the position fordetecting the predetermined position is synchronized with the period ofchange in speed of the machine under a low speed rotation-driving suchthat the machine can be stopped at the predetermined position at anytime.

2. Description of the Prior Art In the field of rotatably drivenmachines, such for example as sewing machines, which are coupled througha clutch and a brake to a rotation-driving source, it is very importantto be capable of stopping the machine at a predetermined position.

According to prior art apparatus of this kind, in order to stop amachine which is running at a high speed at a predetermined position,the machine was first decelerated to a low speed and while maintained atthe low speed, the stopping position was detected during the low speeddriving. Thus, in the past in order to stop a rotatably driven machinethe same had to be maintained at such a low speed driving condition thatthe speed of the machine would periodically change about a predeterminedlow value of speed. Then, by detecting the speed of the machineandcontrolling the clutch and brake coupling, the machine with therotatably driving source could by means of a detection factor, then bebraked for stopping during the low speed driving.

While somewhat satisfactory, with the prior art apparatus, since thespeed of the machine to be stopped would periodically change within arange about a predetermined low value of rotation speed, the detectionof the stopping position would be effected depending upon whether themachine was at the maximum speed or minimum speed. Thus, the timerequired for the effective stopping of the machine when at a maximumspeed will be different, though slightly, from that for effectingstopping when the machine is at minimum speed. For this reason, theposition at which the machine is finally stopped will be differentdepending upon the speed at the time of stopping. Accordingly adisadvantage in the prior art is that the accuracy of a desired stoppingposition is limited by speed variations. Moreover, if the machinestopping position is detected when the machine is at a maximum speed thebrake will be burdened thereby.

SUMMARY OF THE INVENTION Accordingly, it is one object of this inventionto provide a new and improved apparatus for stopping the rotating memberof a driven machine at a predetermined position.

Another object of this invention is to provide a new and improved uniqueapparatus for stopping the rotating member of a driven machine at apredetermined position with a high degree of accuracy and which is bothcompact and inexpensive.

Yet another object of this invention is to provide a new and improvedapparatus for stopping a machine at a predetermined position byutilizing a speed determining device which when the center of themagnetic poles of a rotor thereof is aligned with the stator thereofwill be at the predetermined position.

Briefly, in accordance with this invention the foregoing and otherobjects are in one aspect attained by the use of a pilot generator whichhas a rotor and a stator each of which has a plurality of magnetic polesfor detecting the speed of a rotatably driven machine in combinationwith a position detecting device which rotates simultaneously with therotatably driven machine and which provides the predetermined stoppingposition such that when the center of the magnetic poles of the rotor ofthe pilot generator are aligned with the stator thereof thepredetermined position will be established by the position detectingdevice.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of theinvention will be readily obtained as the same becomes better understoodby reference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a view illustrating an exemplary sewing machine in conjunctionwith the apparatus of the present invention which enables the machine tobe stopped at a predetermined position;

FIG. 2 is a view illustrating in detail a position detecting devicesuitable for use in the embodiment of FIG. I;

- FIG. 3 is a view illustrating in detail how the brushes within theposition detecting device of FIG. 2 make contact with an annular segmenttherein;

FIG. 4 is a view illustrating in detail a driving and controlling devicesuitable for use in the embodiment of FIG. 1;

FIG. 5 is a schematic diagram of an electric controlling device suitablefor use in the embodiment of FIG. I; and

FIG. 6 is a plurality of graphs which illustrate various relationshipswith respect to time of the apparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawings wherein like reference numerals designate identical, orcorresponding parts throughout the several views, and more particularlyto FIG. 1 thereof wherein a sewing machine 10 is shown as including aworking table 11 and a machine body 12 which is mounted on the topsurface of the table 11. The machine body 12 has a base portion 13 whichextends in a vertical direction from the table 11, a drum portion 14which extends in a substantially horizontal direction from the top endof the base portion 13 and a head portion 15 which is provided at theend of the drum portion 14. A main shaft 16 extends through the drumportion 14 and is used for enabling a sewing needle 17 to move up anddown through an eccentric cam mechanism (not shown) which is providedwithin the head portion 15. A pulley 18 is fixed to the main shaft 16. Adevice 20 is provided for detecting the position of the sewing needle 17and the same is coupled to the main shaft 16. The position detectingdevice 20 is shown and described in detail hereinafter with reference toFIG. 2.

Referring now to FIG. 2, an auxiliary shaft 201 is shown and the same isconnected directly to the main shaft 16 and an annular segment 23 whichconsists of an electric conductor 21 dispersed on the peripheral surfaceof the auxiliary shaft 201 and insulating members 22 and 22 provided atportions along the segment. First, second and third brushes 24, 25 and26 are respectively provided in sliding contact with the annular segment23. The first brush 26 is arranged to make contact with the insulatingmember 22 of the annular segment 23 when the sewing needle 17 is in apredetermined position, such for example as the top position within itsstroke of motion. The second brush 25 is arranged to make contact withthe insulating member 22 when the sewing needle 17 is in anotherpredetermined position, such for example as the bottom position withinits stroke of motion. The third brush 24 is arranged to always makecontact with the electrical conductor 21. A casing 27 is provided forsupporting the brushes 24, 25 and 26. A plate 28 (see FIG. 1) isprovided for supporting and connecting the casing 27 to the table 11. Apilot generator 29 is provided for producing an alternating voltageproportional to the speed of rotation of the main shaft 16. As shown inFIG. 2, the pilot generator 29 comprises a stator 202 enclosed by thecasing 27 and a rotor 203 which is coupled to the auxiliary shaft 201.

Furthermore, as clearly shown in FIG. 3, the first and second brushes 26and 25 are arranged to respectively make contact with the insulatingmembers 22 and 22' of the annular segment 23 of the detecting device 20when the center of the magnetic poles N,S of the rotor 203 coincideswith the magnetic poles of the stator 202.

Referring again to FIG. 1, a driving and controlling device 30 is shownas being provided beneath the table 11. The driving and controllingdevice 30 is provided within a housing 31 and is connected beneath thetable 11 by a mounting plate 32.

The driving and controlling device 30 is shown and described in detailwith reference to FIG. 4. In FIG. 4, a motor 310 is used as arotation-driving source. An iron core 311 which forms a part of thestator of the motor 310 is shown as being fixed to the inside peripheryof the above-mentioned housing 31. A stator coil 312 is shown connectedto the iron core 311. A left side bracket 313 is provided at the leftend of the housing 31 and a middle bracket 314 is provided at the centerportion of the housing 31. A rotating shaft 315 of the motor 310 issupported through ball bearings 316 and 317 on the brackets 313 and 314.A rotor iron core 318 is provided and fixed to the rotating shaft 315and supports a rotor coil 319.

A clutch device 320 is provided. A rotating body 321 of the clutchdevice 320 is provided and is mounted on the right end of the rotatingshaft 315. A right-side bracket 324 is provided at the right end of thehousing 31. A sleeve bearing 325 is provided at the center portion ofthe right-side bracket. A movable sleeve bearing 326 is provided andinserted into the central opening of the sleeve bearing 325 and the samemay therefore rotate and axially move therein. An output shaft 327 issupported through ball bearings 328 and 329 on the inside periphery ofthe movable sleeve bearing 326 and is arranged to be coaxial with therotating shaft 315.

A pulley 330 is fixed to the right end of the output shaft 327 by a key331. The pulley 330 is connected to the pulley 18 through a belt 33 asshown by the broken line in FIG. 1. A rotating body 332 of the clutchdevice 320 is provided and is fixed to the left end of the output shaft327. Annular friction plates 335 and 336 are fixed to both sides of therotating body 332. The friction plate 335 is positioned opposite to therotating body 321.

A stationary plate 341 of a brake device 340 is provided and is in theform of a ring and fixed to the inside periphery of the right-sidebracket 324. The plate 341 is positioned opposite to the friction plate336 and serves as the stator of the brake device 340.

An electromagneto device 350 is provided and serves to control both theclutch 320 and the brake 340. The stator 35] of the electromagnetodevice comprises an annular supporting plate 352 fixed on the stationaryplate 341, an annular iron core 353 of a C-type cross section to theinside periphery of the supporting plate 352 and an electromagnetic coil354 is wound on the annular iron core 353. The movable element 355 ofthe electromagneto device 350 comprises an annular movable plate 358loosely fitted onto the periphery of the left end of the movable bearing326 and positioned between stops 356 and 357 and an annular moving ironcore 359 which is fixed onto an axially extending face of the movableplate 358 and positioned opposite to the annular iron core 353.

A guide ring 360 is provided and fixed on the outside periphery of thesleeve bearing 325 and makes contact with the inside periphery of thesleeve portion of the movable plate 358 to guide the movement thereof. Aplurality of screw rods 361 are arranged to pass through the movableplate 358 at peripheral portions thereof. Nuts 362 are provided andscrewed into that part of the screw rods 361 which projects from theright-side bracket 324. Coil springs 363 are provided and are arrangedbetween flanges 364 at the inner side of the screw-rods 361 and themovable plate 358. The coil springs 363 serve to apply forces oppositeto that of the attracting force of the moving iron core 359 to themoving plate 358 which is caused by the electromagnetic coil 354 suchthat the rotating body 332 is detached from the rotating body 321 andthen moved toward the stationary plate 341. An L-type operating lever370 is pivotally mounted on the right-side bracket 324 by a pin 371. Thelever 370 is provided with a forked part 372 at one end thereof. Theforked part 372 is adapted to hold the sleeve bearing 325 between thelegs thereof. A bolt 373 is provided at the end of the forked part 372and is fitted to the elongated aperture 374 of the movable bearing 326to enable the movable bearing 326 to be moved in an axial direction. Ascrew rod 375 is provided and is supported at one end by the right-sidebracket 324. The screw rod 375 passes through the lever 370 at a portionthereof which extends below the pin 371. A nut 376 is screwed into theother end of the screw rod 375. A coil spring 377 is arranged betweenthe nut 376 and the lever 370 and serves to bias the movable bearing 326toward the right side so that the rotating body 332 is disconnected fromthe rotating body 321. A microswitch 380 is provided and is fixed to theouter side of the upper portion of the right-side bracket 324. Anactuating element 381 is provided for the microswitch'380. An operatingmember 382 is provided at one of the legs of the forked part 372 andprojects therefrom so as to engage with the actuating element 381. Thus,when the lever 370 is rotated in a counterclockwise direction about thepin 371 and the spring 377 become biased, the rotating body 332 will beapplied against the rotating body 321 with'a sufficient force so thatthe operating member 382 will depress the actuating element 381 andthereby open the microswitch 380.

Referring again to FIG. 1, it is seen that a pedal 40 of the sewingmachine is provided which is capable of rotating about a shaft 41. Aconnecting bar 42 connects an aperture 378 at the end of the lever 370to the pedal 40. An electric controlling device 50 is provided and ismounted beneath the table 11. The electric power source operating board501 of the controlling device 50 has two push buttons 503 and 504 forswitching a power source switch.

The details of the electric controlling device 50 can best be understoodwith reference to FIG. 5 wherein a lead 505 is shown as connecting thepower source switch 502 to an AC power source 506 and a lead 507 isshown as connecting the power source switch 502 to a motor 310. A singlephase full wave rectifier 508 is provided and a bilateral three-terminalthyristor 509 is connected to the AC input side thereof. A resistor 511is connected to the DC output side of the rectifier 508 and a lead 512connects the resistor 511 to the electromagnetic coil 354.

A condenser 513 and a triggering element such for example as a bilateralthree-layer diode 514 are provided in the gate circuit of the thyristor509. When the voltage across the condenser 513 exceeds a predeterminedabsolute value V0 with the polarity shown in FIG. 5, the three-layerdiode 514 will become conductive and turn on the thyristor 509.Similarly, when the voltage across the condenser 513 exceeds the sameabsolute value V0 with a polarity opposite to that shown in FIG. 5, thethree-layer diode 514 will still turn on the thyristor 509. A reactor516 is provided in series with the condenser 513 and a lead 517 servesto connect the reactor 516 to the microswitch 380. A changeover switch518 is provided for selecting either the first brush 26 or the secondbrush 25 of the position detecting device 20 of the sewing machine. Ifthe changeover switch 518 is placed in the upper position, the firstbrush 26 is selected, while if the switch is placed in the lowerposition, the second brush 25 is selected. Reference number 519collectively indicates leads for connecting both the position detectingdevice 20 and the pilot generator 29 to the electric controlling device50. The third brush 24 of the position detecting device 20 is connectedto the microswitch 380 through one of these leads. Additionally, asingle phase full wave rectifier 520 is provided and connected acrossthe condenser 513. An NPN-type transistor 521 is provided and a resistor 522 is connected between the transistor 521 and the rectifier520. A single phase full wave rectifier 523 is provided and the DCoutput side thereof is connected to the base and emitter of thetransistor 521, while the AC input side thereof is connected to thepilot generator 29. A variable resistor 524 is provided and connected tothe base of the transistor 521. A filter circuit 525 which consists of acondenser 526 and a resistor 527 serves to smooth the DC output from therectifier S23.

The operation of the preferred embodiment of the apparatus of thisinvention as set forth above can be described as follows. The sewingmachine may be turned on by depressing the push button 503 to cause thepower source switch 502 to be closed. The motor 310 is thereby energizedand thus the rotating shaft 315 is driven. Under this condition, if thepedal 40 is stepped on the connecting bar 42 will be pulled and thelever 370 will be rotated in a counterclockwise direction about the pin371 against the spring 377. The movable bearing 326 will be moved towardthe left side by the bolt 373. The movement of the movable bearing 326will thereby cause the output shaft 327 together with the ball bearings328 and 329 to be pushed toward the left side. The friction plate 335 ofthe rotating body 332 will then be pressed against the rotating body 321and thereby drive the rotating body 322. The clutch 320 will thenoperate without causing any substantial slip between the rotating bodies321 and 332. Thus, the output shaft 327 will be driven at a rotationspeed substantially equal to that of the output shaft 315. The mainshaft 16 is driven through the pulleys 330 and 18. Accordingly, thesewing needle 17 will be caused to move up and down to enable a sewingoperation to be effected. Generally, by way of example, the main shaft16 may be driven at a speed of 3,000 to 5,000 rpm. It should beunderstood that the rotation of the lever 370 will cause the microswitch380 to be actuated and thereby open the contact of the microswitch.Under this condition the condenser 513 of FIG. 5 will not be charged. Assuch, the thyristor 509 will be in the off condition and no electriccurrent will flow through the electromagnetic coil 354 of theelectromagnetic device 350.

In order to stop the sewing machine 10 the force which has been appliedto the pedal 40 will be removed. Under such conditions, rotating body332 will be moved toward the right side by the spring 363, an'd'theclutch 320 will be released. The friction plate 336 of the rotating body332 will be pressed on the stationary plate 341 of the brake device 340.Since the clockwise rotation of the lever 370 is prevented after therotating body 332 is detached from the rotating body 321, the spring 377will no longer be able to move the rotating body 332 further towards theright side. The spring 363, however, will move the rotating body 332towards the right side by the resulting clearance between the elongate374 and the bolt 373. Thus, the main shaft 16 which would continue torotate by reason of inertia will be braked and as such the rotationspeed of the main shaft 16 will rapidly be reduced.

By the returning operation of the lever 370, the contact of themicroswitch 380 will now be closed. Now, if it is assumed that thechangeover switch 518 has been thrown into the upper position then thefirst brush 26 of the position detecting device 20 will make contactwith the conductor 21 and the series circuit which includes thechangeover switch 518, the first brush 24, the conductor 21, and thethird brush 26, the microswitch 380, the reactor 516 and the condenser513 will be completed. However, if the speed of rotation of the mainshaft 16 is considerably high then the output voltage from the pilotgenerator 29 will be above a predetermined level and a base current willflow through the base and emitter of the transistor 521 from therectifier 523 and will be sufficiently large to make thecollector-emitter of the transistor 521 fully conductive. An electriccurrent from the power source 506 of a voltage e, of the polarity, asshown by solid line arrow in FIG. 5, will flow through a circuitincluding the transistor 521, the resistor 522 and the rectifier 520.The condenser 513 will be bypassed. Similarly, an electric current fromthe power source 506 of a voltage e of the polarity, as shown by brokenline arrow in FIG. 5, will flow through a circuit including therectifier 520. Again the condenser 513 will be bypassed. Thus,regardless of the polarity of the voltage of the power source 506 thecondenser 513 will not be charged to a voltage level sufficient to makethe triggering element 514 conductive and as such the thyristor 509 willnot be turned on. In turn, the electromagnetic coil 354 of theelectromagneto device 350 will not be energized. The friction plate 336of the rotating body 332 will still be pressed on the stationary plate341 and the main shaft 16 will continue to be decelerated.

It should be understood that when the speed of rotation of the mainshaft 16 is reduced to a predetermined value, the output voltage of thepilot generator 29 will be reduced in proportion thereto. At such timethe conductivity of the transistor 52] will change and the resistancebetween the collector and the emitter of the transistor 521 willincrease. This increase in the collector-emitter resistance will lessenthe bypassing function of the transistor 521 so that the condenser 513will be charged to a sufficient voltage to cause the triggering element514 to conduct. If the polarity of the voltage of the power source 506is as shown by e. then the condenser 513 will charge with the polarityas shown in FIG. 5. As soon as the condenser 513 is charged to thepredetermined absolute value Vo, the thyristor 509 will be turned on. Ifthe polarity of the voltage of the power source 506 is as shown by ethen the condenser 513 will charge with a polarity opposite to thatshown in FIG. 5. Again, as soon as the condenser 513 is charged to thepredetermined absolute value Vo, the thyristor 509 will be turned on.The rectifier 508 will rectify the AC input wave applied thereto inaccordance with the firing phase angle of the thyristor 509. Theresultant rectified output will then be applied to the electromagneticcoil 354. As the output voltage of the pilot generator 29 is lowered,the resistance of the transistor 521 will increase and the condenser 513will charge faster so that the thyristor 509 is fired at earlier phase.Under such conditions, the duration of the AC signal as obtained by thethyristor 509 will be longer and thus the electric current flowingthrough the electromagnetic coil 354 will be increased. When theelectric current flowing through the electromagnetic coil 354 is soincreased, the force attracting the moving iron core 359 by the annulariron core 353 will exceed that of the springs 363 and the rotating body332 will be caused to detach from the stationary plate 341 and againmake contact with the rotating body 321. The clutch device 320 willagain be coupled. However, since in this case the force pressing thefriction plate 335 of the rotating body 332 against the rotating body321 will change in proportion to the electric current flowing throughthe electromagnetic coil 354 and not be sufficiently strong, the outputshaft 327 will be driven with a slip being caused between the rotatingbody 321 and the friction plate 335. That is, the output shaft 327 willbe driven in a so-called semiclutching condition. Because the torquetransferred in this semiclutching condition will increase as the speedof rotation of the main shaft 16 is reduced, the main shaft 16 will becontinuously driven with the speed of rotation thereof periodicallychanging about a predetermined low level, such for example as 200 rpm.

During the continuous driving condition, the period of time for whichthe first brush 26 will continue to contact the insulating member 22will be sufficiently long so as to prevent the condenser 513 from beingcharged during a rest period of time which is sufficient to cause theelectromagnetic coil 354 to become fully deenergized. When theelectromagnetic coil 354 is deenergized, the rotating body 332 willagain be pressed against the stationary plate 341 for braking.Accordingly, the rotation of the main shaft 16 will stop. The sewingmachine 10 can thus be stopped in the position at which the insulatingmember 22 makes contact with the first brush 26, that is, in the upperposition of its moving stroke. The continuous driving condition and theprocess of stopping can be further described in greater detailhereinafter in conjunction with FIG. 6.

Referring now to FIG. 6, the graph (0) illustrates the relationshipbetween the changing positions of the stator 202 and the rotor 203 ofthe pilot generator 29 as time elapses. The graph (b) shows the outputvoltage of the pilot generator 29 plotted against time. The graph (0)shows the base current flowing to the transistor S21 plotted againsttime. The graph (d) shows the AC signal applied to the rectifier 508 inaccordance with the firing phase angle of the thyristor 509. The graph(e) shows the electric current flowing through the electromagnetic coil354 plotted against time. The graph (f) shows the speed of rotation ofthe main shaft 16 which is controlled by the current, shown in graph(e), and continuously driven at a predetermined low level. The graph (g)illustrates the relationship between the positions of the insulatingmember 22 and the first brush 26 of the position detecting device 20. Itshould be understood that all of the graphs (a) through (g) coincidewith each other with respect to the timecalibrated along the axis ofabscissa.

It is well known that even if the pilot generator 29 is rotated at aconstant speed while in the continuous driving condition that the samewill produce a sinusoidal AC voltage which will have a zero value uponalignment of the magnetic poles of the stator 202 and the rotor 203 anda maximum value when the magnetic poles of the stator and the rotordiffer by an electrical angle of 90, as shown in graphs (a) and (b).Thus, the base current of the transistor 521 is proportional to theoutput voltage and is shaped by the filter circuit into a wave form asshown in graph The voltage wave form applied to the rectifier 508 is asshown in graph (d) for the following reason. As mentioned above, whenthe base current is high, the thyristor 509 will be fired at a laterphase while when the base current is low, the thyristor 509 will befired at an earlier phase. Accordingly, as shown in graph (e), theelectric current flowing through the the electromagnetic coil 354 willbecome lower when the base current is high and become higher when thebase current is low. On the other hand, because of delays in response ofthe components, the speed of rotation of the main shaft 16 will be asshown in graph (f). The main shaft 16 will be continuously driven withthe speed of rotation thereof periodically changing about apredetermined value in such a manner that the speed will become highwhen the base current is high and the speed will become low when thebase current is low. In other words, the speed of rotation of the mainshaft 16 is at a minimum value when the magnetic poles of the stator 202and the rotor 203 of the pilot generator 29 are aligned with each other,and will be of a maximum value when the magnetic poles differ by anelectrical angle of 90. in such a manner, the speed of rotation of themain shaft 16 will periodically change.

According to this invention, because the insulating members 22 or 22' ofthe position detecting device are arranged to contact with the first orsecond brush 26 or thereof when the center of the magnetic pole of therotor 203 of the pilot generator is aligned with that of the stator 202thereof then the contact of the insulating members 22 or 22' with thefirst or second brush 26 or 25 and thus the stopping at a predeterminedposition will be effected exactly at the time when the speed of rotationis lower, as shown in graph (g). This enables an easy braking. Thus, itshould be apparent that according to this invention machine stoppingsare not effected when the machine is at different speeds during thecontinuous driving condition, as in the past, but will always beeffected when the machine is at the same speed. With such a uniqueadvantage it is possible not only to widely improve the reliability andrapidity of stopping of the machine, but also to greatly improve theaccuracy of the stopping position.

It should be understood that although this invention has been describedwith respect to an embodiment wherein a sewing machine is used as arotatably driven machine and that an electric motor has been used as therotation-driving source, that the invention is not so limited and may beapplied to all other rotatably driven machines which are desired to bestopped at a predetermined position of rotation. Thus, for example, thepresent invention is equally applicable to riveting machines wherein ahammer is moved up and down through rotatably driving of the same,sawing machines wherein a fret saw is moved up and down throughrotatably driving of the same or the like.

It should now be apparent that according to this invention the magneticpoles of a pilot generator are adapted to detect the speed of a machinein order to effect a low speed driving through an automaticsemiclutching operation. With the present invention, since machinestopping can always be effected under the same low speed conditionswhile the low speed rotation-driving is maintained in an oscillatingmanner, the stopping at a predetermined position can be rapidly andsurely effected with the accuracy thereof greatly improved. In aparticular application, it has been found that the accuracy of machinestopping in accordance with the teachings of this invention was one anda half times better than that of the prior art.

Moreover it should be apparent that since the magnetic attracting forceacting between the stator and the rotor of the pilot generator of thisinvention at a maximum value when the magnetic poles of the stator andthe rotor are aligned with each other and thereby provide a brakingforce that the accuracy of machine stopping will be further improved byutilization of such braking force.

Obviously, numerous modifications and variations of the presentinvention are possible in light-'of the above teachings. It shouldtherefore be understood that within the-scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. An apparatus for stopping a rotatably driven machine at apredetermined position comprising:

a source of power for driving said rotatably driven machine;

a pilot generator which has a rotor and a stator, each of which has aplurality of magnetic poles for detecting the speed of said rotatablydriven machine and for generating a fluctuating voltage causing saidrotatably driven machine to be driven at a speed which variesperiodically about a predetermined value; and,

a position detecting device which rotates simultaneously with saidrotatably driven machine for determining said predetermined stoppingposition of said machine in accordance with said periodically varyingspeed.

2. An apparatus as claimed in claim 1, wherein the predeterminedstopping position of said position detecting device is a position suchthat the center of the magnetic poles of said rotor of said pilotgenerator is aligned with that of the stator thereof.

3. An apparatus as claimed in claim 2, wherein said rotatably drivenmachine includes a rotating shaft and wherein said position detectingdevice further comprises:

an auxiliary shaft connected to the rotating shaft of said rotatablydriven machine;

an annular segment provided on said auxiliary shaft, the

peripheral surface of which includes an electrical conductor andinsulating members;

a plurality of brushes each of which slide-contacts with said annularsegment; and v a housing for supporting said brushes.

4. An apparatus as claimed in claim 3, wherein the rotor of said pilotgenerator is connected to said auxiliary shaft of said positiondetecting device and wherein the stator of said pilot generator ispositioned opposite to the peripheral surface of said rotor and issupported by said housing.

5. An apparatus as claimed in claim 4, wherein the insulat ing membersof said position detecting device make contact with the brushes thereofwhen the center of the magnetic poles of the rotor of said pilotgenerator is aligned with that of the stator thereof.

1. An apparatus for stopping a rotatably driven machine at apredetermined position comprising: a source of power for driving saidrotatably driven machine; a pilot generator which has a rotor and astator, each of which has a plurality of magnetic poles for detectingthe speed of said rotatably driven machine and for generating afluctuating voltage causing said rotatably driven machine to be drivenat a speed which varies periodically about a predetermined value; and, aposition detecting device which rotates simultaneously with saidrotatably driven machine for determining said predetermined stoppingposition of said machine in accordance with said periodically varyingspeed.
 2. An apparatus as claimed in claim 1, wherein the predeterminedstopping position of said position detecting device is a position suchthat the center of the magnetic poles of said rotor of said pilotgenerator is aligned with that of the stator thereof.
 3. An apparatus asclaimed in claim 2, wherein said rotatably driven machine includes arotating shaft and wherein said position detecting device furthercomprises: an auxiliary shaft connected to the rotating shaft of saidrotatably driven machine; an annular segment provided on said auxiliaryshaft, the peripheral surface of which includes an electrical conductorand insulating members; a plurality of brushes each of whichslide-contacts with said annular segment; and a housing for supportingsaid brushes.
 4. An apparatus as claimed in claim 3, wherein the rotorof said pilot generator is connected to said auxiliary shaft of saidposition detecting device and wherein the stator of said pilot generatoris positioned opposite to the peripheral surface of said rotor and issupported by said housing.
 5. An apparatus as claimed in claim 4,wherein the insulating members of said position detecting device makecontact with the brushes thereof when the center of the magnetic polesof the rotor of said pilot generator is aligned with that of the statorthereof.